The present invention relates in general to an ignition system for an internal combustion engine and in particular to an ignition system of a current interruption type in which the primary current in an ignition coil supplied from an a.c. power source is interrupted by a current interrupting element under the control of a suitable control circuit.
Hitherto, there have been generally known a so-called battery ignition system and a magnet ignition system as the current interruption type ignition system for the internal combustion engine of a motor bicycle which is equipped with an a.c. generator.
In the case of the battery ignition system, the carried battery is charged by the output power produced by the a.c. generator and a d.c. current from the battery is supplied to the ignition coil. By interrupting the d.c. current directly through the interrupter, a high voltage is induced in the secondary winding of the ignition coil. As an improved battery ignition system of such a current interruption type, there has been lately proposed a transistorized system, according to which the current from the battery is supplied to the ignition coil through a switching transistor which is controlled by a small current in synchronism with the ignition cycle of the internal combustion engine thereby to interrupt the d.c. current fed to the primary winding of the ignition coil. The ignition system in which the conventional interrupter is employed as a means for producing such a small current for controlling the switching transistor is referred to as the contact-type transistorized ignition system or semi-transitorized ignition system. On the other hand, instead of utilizing such an interrupter, it is known to use for the same purpose an electromagnetic pick-up device comprising a rotatable permanent magnet device provided with magnetic poles in number corresponding to that of the cylinders of the engine and a sensor constituted by a detection coil wound around a core and positioned in opposition to the rotatable magnet device so that the electromotive force is induced in the detection coil in response to the passage of the magnetic poles. This system is often referred to as the contactless-type transistorized ignition system or full-transistorized ignition system.
On the other hand, an ignition system of a fly-wheel type is known as a variety of the magnet type ignition system, in which a so called fly-wheel magnet serving simultaneously as the fly-wheel for the engine constitutes an a.c. generator to supply an a.c. current directly to the ignition coil, which current is interrupted directly by an interrupter thereby to induce a high voltage in the secondary coil of the ignition coil.
In addition to the ignition system of the current interruption type, there has been proposed an ignition system of a capacitor discharge type in which the output current of the a.c. generator of the magnet rotor type or the fly-wheel magnet type is adapted to be temporarily stored in a capacitor which is abruptly discharged through the primary coil of the ignition coil by means of a thyristor enabled by a gate pulse signal. Such an ignition system in which the gate pulse signal is produced by an interrupter is termed as a contact type, while such an ignition system in which an electro-magnetic pick-up device is employed is termed as the contactless type.
In the ignition systems in which the primary current flowing in the ignition coil is directly interrupted by an interrupter, it is known that the contacts of the interrupter are likely to be burnt off due to the produced arc and can not be evaded from the migration phenomenon, involving the lowered voltage induced in the secondary winding of the ignition coil as well as failures in the spark generation, even if the contacts are made of tungsten or the like material. Thus, frequent repair of the contacts is required.
Besides, since the interrupting operation of the interrupter is mechanically controlled by a cam means, there will arise a chattering phenomenon of the contacts of the interrupter at high revolution speeds of the engine even in the case of the capacitor discharge ignition system of the contact type or the semitransistorized type in which the primary current is not interrupted directly. Such chattering phenomenon will of course give rise to an abrupt decrease in the induced secondary voltage.
In view of the disadvantages of the prior art described above, the contactless type ignition system is preferred. However, in order to accomplish such a contactless type ignition system whether of the falltransistorized current interruption variety or the capacitor discharge type, a pulse generator means for producing the gate or control pulse in synchronism with the ignition cycle of the engine is indispensably required. Such pulse generator means which functions properly can not, however, be realized in the case of the motor bicycle not carrying a battery because the output voltage of the generator alternates. For this reason, practical ignition systems of the contactless type have not yet been brought into use at the present technical state.